Electrochemical Conversion of Hydrocarbons

ABSTRACT

An electrochemical conversion method for converting at least a portion of a first mixture comprising hydrocarbon to C 2+  unsaturates by repeatedly applying an electric potential difference, V(τ 1 ), to a first electrode of an electrochemical cell during a first time interval τ 1 ; and reducing the electric potential difference, V(τ 1 ), to a second electric potential difference, V(τ 2 ), for a second time interval τ 2 , wherein τ 2 ≦τ 1 . The method is beneficial, among other things, for reducing coke formation in the electrochemical production of C 2+  unsaturates in an electrochemical cell. Accordingly, a method of reducing coke formation in the electrochemical conversion of such mixtures and a method for electrochemically converting carbon to C 2+  unsaturates as well as an apparatus for such methods are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/898,066, filed Oct. 31, 2013, and EP 14151211.1 filedJan. 15, 2014 the disclosures of which are fully incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to electrochemical conversion ofhydrocarbon-containing feed streams. In particular, embodiments of theinvention relate to electrochemical conversion of methane to higherhydrocarbons, particularly ethylene.

BACKGROUND OF THE INVENTION

Electrochemical conversion of methane to higher hydrocarbon molecules,e.g., C₂₊ saturated hydrocarbons and C₂₊ unsaturated hydrocarbons isknown. For example, the electrochemical conversion of methane has beenaccomplished in an electrocatalytic cell comprising an anode chamber, acathode chamber, the anode chamber being separated from the cathodechamber by a membrane which prevents the flow of atoms, molecules, andelectrons between the anode and cathode chambers, but permits the flowof ions. See, e.g., Chiang, et al., J. Eletrochem. Soc. 138, 6, L11, 12(1991) and Hasakawa, et al., J. Electrochem. Soc. 140, 2, 459-462(1993). In a conventional electrochemical cell, the anode chambercontains an anode in diffusive and electrical contact with the membrane,and the cathode chamber contains a cathode in diffusive and electricalcontact with the membrane. The anode and cathode can be electricallyconnected via an external circuit, and the cell can be operated ineither constant electric current or constant voltage mode.

In such a cell, methane may be conveyed to the anode chamber where thecatalyst activates the methane to form CH₃ fragments, a hydrogen ion(H⁺), and a free electron (e⁻). The hydrogen ions transit the membrane,and at the cathode produce a product comprising, e.g., molecularhydrogen in systems where oxygen is absent, utilizing the free electronsconducted from the anode to the cathode via the external circuit. Insystems where oxygen is present, a product comprising water is producedat the cathode. In either case, the desired C₂₊ unsaturates areconducted away from the anode chamber.

In addition to activating the methane, the anode can further oxidize theCH₃ fragments produced by the activation, resulting in the production ofsurface-bound CH₂ fragments, which can in turn be oxidized to CHfragments. The CH fragments can be further oxidized to form carbon. Suchcarbon formation on the anode is the dominant reaction, resulting in arelatively small amount of C₂₊ hydrocarbon being produced by the cell.

Methods of increasing methane conversion to C₂₊ hydrocarbon have beendescribed. For example, the use of an inorganic membrane in the cellenables the use of relatively high temperatures, thereby shiftingequilibrium toward the formation of ethylene. The increase in ethylene,however, is accompanied by a loss in overall cell efficiency and anincrease in carbon accumulation on the anode.

It is desired to increase the relative amount of C₂₊ hydrocarbonproduced at the cell's anode, and in particular the relative amount ofethylene, while maintaining cell efficiency and lessening the amount ofcarbon accumulating on the cell's anode.

SUMMARY OF THE INVENTION

It has been observed that the electrode's active sites forelectrocatalytic conversion of C₁₊ hydrocarbon (e.g., C₁₊ alkane, suchas methane) to C₂₊ unsaturates are the same sites that are active forsuccessive hydrocarbon oxidation. Successive hydrocarbon oxidation canlead to undesirable carbon formation on the electrode. It has beendiscovered that carbon formation can be mitigated, or even reversed, ifthe electric potential difference between the positive and negativeelectrodes of the cell is decreased, reversed in polarity, or even madesubstantially equal to zero for intervals during the conversion process.Further, in particular embodiments, the cell's operating conditions(e.g., electrode composition, voltage, current density etc.) areselected to favor the selection of C—C bond formation pathways overnon-productive pathways.

Certain aspects of the invention provide an electrochemical conversionmethod carried out using an electrochemical cell. The electrochemicalcell includes a first electrode, a second electrode, and at least onemembrane located between the first and second electrodes. The processutilizes a first mixture as a feed to the cell's first electrode. Thefirst mixture comprises hydrocarbon, e.g., methane. The first and secondelectrodes can be in physical contact with the membrane, and aregenerally located on opposite side of the membrane. The membrane can beone that is substantially non-porous, but allows for the flow of ionsbetween the first and second electrodes. The process includes applyingan electric potential to the cell to establish an electric potentialdifference, V₁, between the first and second electrodes (conventionallydescribed as establishing an electric potential difference V₁ across thecell) during a first time interval τ₁; and then changing the appliedelectric potential to establish a second electric potential difference,V₂ for a second time interval τ₂. Generally, τ₂ is less than or equal toτ₁, e.g., τ₂ can be in the range of from 0.01·τ₁ to 0.1·τ₁. Generally,V₂ is more negative (or less positive) than V₁. Optionally, V₁ and V₂are related as follows: (i) V₁>0, V₂≧0, and V₂=A₁·V₁, where A₁ is in therange of from about 0.00 to about 0.99; (ii) V₁≧0 and V₂<0; or (iii)V₁<0, V₂<0, and |V₂|=A₂|V₁|, where A₂≧1.01. Optionally, V₁≧0, and|V₂|<|V₁|.

In operation, the cell produces a second mixture comprising C₂₊unsaturates in the vicinity of the second electrode during at least thesecond interval, and generally during the first and second interval. Thesecond electric potential difference V₂ is typically selected so thatC₂₊ unsaturates can form from CH₂ fragments, but at a diminished rate ofCH₂ oxidation to CH fragments and carbon compared to the rate at V₁.

After the second interval the applied electric potential can be changedagain, e.g., to establish a third electric potential difference V₃between the first and second electrodes for a third time interval τ₃.Optionally, (i) V₃ is substantially the same as V₁ and/or (ii) τ₃ issubstantially the same as τ₁. For example, V₂ can be more negative (orless positive) than V₃. Optionally, V₃ and V₂ are related as follows:(i) V₃>0, V₂≧0, and V₂=A₃·V₃, where A₃ is in the range of from about0.00 to about 0.99; (ii) V₃≧0 and V₂<0; or (iii) V₃<0, V₂<0, and|V₂|=A₄|V₃|, where A₄≧1.01. Optionally, V₁≧0, and |V₂|<|V₃|.

After the third interval the applied electric potential can be changedagain, e.g., to establish a fourth electric potential difference V₄between the first and second electrodes for a third time interval τ₄.Optionally, (i) V₄ is substantially the same as V₂ and/or (ii) τ₄ issubstantially the same as τ₂. For example, generally V₄ is more negativeor less positive than V₃. Optionally, V₃ and V₄ are related as follows:(i) V₃>0, V₄≧0, and V₄=A₅·V₃, where A₅ is in the range of from about0.00 to about 0.99; (ii) V₃≧0 and V₄<0; or (iii) V₃<0, V₄<0, and|V₄|=A₆|V₃|, where A₆≧1.01. Optionally, V₃≧0, and |V₄|<|V₃|.

Optionally, the process is carried out continuously by conducting “n”changes in sequence to the applied electric potential to establishelectric potential differences V_(n) across the cell for a time intervalτ_(n), where n is an integer ≧3 corresponding to the number of times theapplied electric potential is changed to produce the specified electricpotential differences across the cell for the specified time interval.For example, the process can be carried out by (a) changing the appliedvoltage in sequence, e.g., by establishing a first electric potentialdifference V₁ across the cell for a first time interval τ₁; (b) changingthe applied electric potential to establish a second electric potentialdifference V₂ across the cell for a second time interval τ₂, (c)changing the applied electric potential to re-establish the firstelectric potential difference V₁ across the cell for third time intervalof duration τ₁, and then (d) changing the applied electric potential tore-establish the second electric potential difference V₂ across the cellfor a fourth time interval of duration τ₂. The process can be operatedcontinuously by repeating steps (a)-(d).

Other aspects of the invention provide a method for reducing cokeformation in the electrochemical production of C₂₊ unsaturates in anelectrochemical cell comprising a first electrode, a second electrode,and at least one membrane situated between the first and secondelectrodes, the method comprising: (a) providing a first mixture to thefirst electrode, the first mixture comprising hydrocarbon, e.g.,methane; (b) applying an electric potential to the cell to establish anelectric potential difference, V₁, across the electrochemical cellsufficient to initiate the conversion of at least a portion of thehydrocarbon during a first time interval τ₁; (c) establishing a secondelectric potential difference, V₂, across the cell for a second timeinterval τ₂, wherein τ₂≦τ₁; repeating steps (b) and (c), and producingC₂₊ unsaturates from the hydrocarbon in the vicinity of at least one ofthe first or second electrodes. The specified electric potentialdifferences can be established by, e.g., one or more of: (i) changingthe electric potential applied to the cell from a first applied electricpotential to a second applied electric potential, (ii) by changing thecomposition and amounts of one or more feed components supplied to thecell, (iii) changing the composition and amounts of one or more productcomponents conducted away from the cell, or (iv) changing one or more ofthe cell operating parameters such as one or more of the temperatureand/or pressure at locations within the cell. V₁, V₂, τ₁ and τ₂ can havethe same values as specified in any of the preceding aspects.

In other aspects, the invention provides a method for electrochemicallyconverting carbon to C₂₊ unsaturates comprising: (a) providing anelectrochemical cell, the electrochemical cell having carbon depositedon the first electrode, a second electrode, and a membrane locatedbetween the first and second electrodes; (b) applying an electricpotential to the cell to establish an electric potential differenceacross the cell during a first time interval τ₁ sufficient forconverting at least a portion of the deposited carbon to C₁₊hydrocarbon, such as methane; (c) changing the applied electricpotential to establish a second electric potential difference, V₂,across the cell for a second time interval τ₂ and producing C₂₊unsaturates from at least a portion of the C₁₊ hydrocarbon produced instep (b); and (d) repeating steps (b) and (c). V₁, V₂, τ₁ and τ₂ canhave the same values as specified in the preceding embodiments.Optionally, (i) at least a portion of the deposited carbon is graphiticcarbon formed by the oxidation of hydrocarbon (such as methane)proximate to the first electrode and/or (ii) at least a portion of theC₂₊ unsaturates produced during step (c) are formed from hydrocarbonproduced during step (b).

Other aspects concern an apparatus for the electrochemical production ofC₂₊ unsaturates comprising: (a) an electrochemical cell comprising: (i)a first electrode chamber comprising first electrode, said firstelectrode chamber configured to receive a first mixture comprising ≧1.0wt. % of at least one hydrocarbon based on the weight of the firstmixture; (ii) a second electrode chamber comprising a second electrode,said second electrode chamber configured to receive an oxygen-containingmixture comprising ≧10.0 wt. % of at least one oxidant, based on theweight of the oxygen-containing mixture; and (iii) a membrane configuredto locate between the first and second electrode, the membrane beingsubstantially non-porous, but that is open to ion transport between thefirst and second electrodes; (b) means for applying an electricpotential to the cell, e.g., a battery or a power supply in electricalcommunication with the electrochemical cell; and (c) means forregulating the electric potential applied to the cell, e.g., a voltageregulator configured to establish a first electric potential differenceacross the cell, V₁, for a first time interval τ₁ and a second electricpotential difference across the cell, V₂, for a second time interval τ₂.V₁, V₂, τ₁ and τ₂ can have the same values as specified in the precedingembodiments. The apparatus can be utilized for carrying out any of theprocesses described in connection with the preceding aspects.Optionally, the hydrocarbon comprises methane. Optionally, the oxygencontaining mixture comprises oxidant such as molecular oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a schematically illustrates a circuit 100 for applying anelectric potential and establishing an electric potential difference.FIG. 1 b schematically illustrates a circuit for applying an electricpotential to an electrochemical cell to establish an electric potentialdifference across the cell. FIG. 1 c schematically shows examples ofelectric potential waveforms that may be applied to the cell.

FIG. 2 schematically illustrates the competing reactions of variousspecies in the electrocatalytic conversion of surface bound CH₃fragments.

FIG. 3 schematically illustrates the competing reactions of variousspecies in the electrocatalytic conversion of surface bound CH₂fragments.

FIG. 4 schematically illustrates the competing reactions of variousspecies in the electrocatalytic conversion of surface bound CHfragments.

FIG. 5 schematically illustrates an apparatus for the electrocatalyticconversion of hydrocarbons according to the invention.

FIG. 6 schematically illustrates an electrochemical cell for use in theelectrocatalytic conversion of hydrocarbons according to the invention.

FIG. 7 schematically illustrates an alternative apparatus for theelectrochemical conversion of hydrocarbons according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

It has been observed that the successive oxidation of C₁₊ hydrocarbon(such as methane) to carbon at an electrochemical cell's electrode,typically the anode, results from establishing an electrochemicalpotential V₁ across the cell for a time ≧τ_(c). Thus, it is observedthat in the electrocatalytic conversion of hydrocarbon such as methane,when the cell is operated for a time, τ_(min), that is less than τ_(c)(e.g., τ≦0.1·τ_(c), or τ≦0.01·τ_(c)), there is insufficient time tosuccessively oxidize the methane to carbon (e.g., coke) on the cell'selectrode. Consequently, if the electric potential difference betweenthe positive and negative electrodes of the cell is decreased, reversedin polarity, or even made substantially equal to zero, C₂₊ unsaturatessuch as ethylene can preferentially form in the cell's electrode chamberfrom CH₂ fragments. Since at least a portion of the CH₂ fragments havebeen consumed, fewer of these fragments remain at the electrode foroxidation to CH, and thereafter to carbon, as might occur when theinitial electric potential difference is restored. In other words,changing the electric potential difference applied to the cell toproduce a change in the electric potential difference across the cellfor a time interval during the conversion lessens the successiveoxidation of methane molecules to carbon atoms and allows theinteraction of the CH₂ fragments to form the desired C₂₊ unsaturates.This can be accomplished by (i) lessening an externally applied voltage,such as by decreasing an external voltage applied to the cell's negativeelectrode thereby making the voltage at the cell's negative electrodeless positive with respect to the voltage at the cell's positiveelectrode and/or (ii) changing the one or more chemical components ofthe anode and/or cathode chamber.

The formation of desirable C₂₊ unsaturates can be increased by selectingoperating conditions that avoid other undesirable reactions shown inFIGS. 2-4. FIG. 2 illustrates the competing reaction pathways in theformation of various derivatives of CH₃ fragments on the electrodesurface, where an asterisk indicates a surface-bound moiety. As FIG. 2shows, once the CH₃ fragment forms, C—C formation is only one of manyoptions for its continued reaction. One such option is the formation ofCH₂ bound species, which can further react along one or more pathways,as FIG. 3 shows. As in the case of surface-bound CH₃ species, C—C bondformation may be more probable than with other pathways in the presenceof CH₂, CH₃ and H fragments. It is believed that once the surface-boundCH fragment forms, the further oxidation to carbon depicted in FIG. 4 ishighly favored on most electrode surfaces. It has been observed thatmitigating CH fragment formation impedes the successive oxidationprocess and mitigates coke formation on the electrode.

The invention is applicable even in embodiments where an undesirableamount of CH fragmentation occurs, leading to carbon forming on at leastone of the cell's electrodes. It has been observed that carbon on anelectrode can be upgraded, i.e., converted to hydrocarbons including C₂₊unsaturates by establishing a sufficient electric potential difference,e.g., V₁, particularly in the absence of more reactive carbon-sourcese.g., methane. Accordingly, in certain embodiments the flow of methaneto the cell is lessened or halted when an electric potential differenceV₁ is established across the cell.

Certain aspects of the invention will be described where the electricpotential difference across the cell (or the current flowing through thecell) are established by applying a first electric potential to the cellduring a first interval of duration τ₁ and applying a second electricpotential to the cell during a second interval τ₂. The invention is notlimited to these aspects, and this description is not meant to forecloseother aspects within the broader scope of the invention, such as thosewhere the first and/or second electric potential difference across thecell are established by one or more of: (i) changing the composition andamounts of one or more feed components supplied to the cell, (ii)changing the composition and amounts of one or more product componentsconducted away from the cell, and/or (iii) changing one or more of thecell operating parameters, such as one or more of the temperature and/orpressure at locations within the cell (e.g., at one or more of theanode, cathode, membrane, anode chamber, cathode chamber, cell walls,etc.).

Certain aspects of the invention relate to an electrochemical conversionprocess. The process comprises providing an electrochemical cellcomprising first electrode, a second electrode, and a membrane. Themembrane, which is typically non-porous, is situated between the firstand second electrodes. In the process, a first mixture comprisinghydrocarbon, e.g., methane, is provided to the first electrode of theelectrochemical cell. An electric potential is applied to the cell toestablish an electric potential difference, V₁, across theelectrochemical cell during a first time interval τ₁. This producescarbon on the cell's first electrode at an average rate of R₁ (moles persecond) during the first time interval. Next, the applied electricpotential is changed to establish a second electrochemical difference,V₂, across the electrochemical cell for a second time interval τ₂. Thisproduces carbon on the first electrode at a rate R₂ during the secondtime interval. This also produces a second mixture comprising C₂₊unsaturates. In these aspects of the invention R₂<R₁ and, preferably,R₂=B·R₁, where B is ≦1.0, e.g., in the range of from about 0.01 to about0.99, such as 0.1 to about 0.5. V₁, V₂, τ₁ and τ₂ can have, e.g., thesame values as specified in any of the preceding aspects. Optionally,the C₂₊ unsaturates are produced during the first time interval.

The first mixture comprises hydrocarbon, e.g., ≧1.0 wt. % of C₁₊hydrocarbon, based on the weight of the first mixture, such as ≧25.0 wt.%, or ≧50.0 wt. %, or ≧75.0 wt. %, or ≧90.0 wt. %, or ≧99.0 wt. %. Forexample, the first mixture can comprise ≧1.0 wt. % of C₁₊ alkane, basedon the weight of the first mixture, such as ≧25.0 wt. %, or ≧50.0 wt. %,or ≧75.0 wt. %, or ≧90.0 wt. %, or ≧99.0 wt. %. The first mixture cancomprise methane, e.g., ≧1.0 wt. % of methane, based on the weight ofthe first mixture, such as ≧25.0 wt. %, or ≧50.0 wt. %, or ≧75.0 wt. %,or ≧90.0 wt. %, or ≧99.0 wt. %. Optionally, the first mixture comprisesdiluent, e.g., one or more of nitrogen, helium, argon, etc.

One or more electrochemical cells is utilized for converting at least aportion of the first mixture to a second mixture comprising C₂₊unsaturates. The remainder of the second mixture can be molecularhydrogen and/or ethane, for example. In certain aspects, ≧0.1 wt. % ofthe first mixture's hydrocarbon is converted to C₂₊ unsaturates of thesecond mixture, particularly to C₂₊ olefin, e.g., ≧1.0 wt. %, such as≧10.0 wt. %. In particular aspects, the first mixture comprises methaneand the second mixture comprises ethylene, e.g., the first mixturecomprises ≧90.0 wt. % methane, such as ≧99.0 wt. % methane, and (ii) thesecond mixture comprises ≧90.0 wt. % ethylene, such as ≧99.0 wt. %ethylene.

The term “C_(n)” hydrocarbon wherein n is a positive integer, e.g., 1,2, 3, 4, or 5, means hydrocarbon having n carbon atom(s) per molecule.The term “hydrocarbon” means compounds containing hydrogen bound tocarbon, and encompasses (i) saturated hydrocarbon, (ii) unsaturatedhydrocarbon, and (iii) mixtures of hydrocarbons, including mixtures ofhydrocarbons (saturated and/or unsaturated) having different values ofn. The term “C_(n+)” hydrocarbon wherein n is a positive integer, e.g.,1, 2, 3, 4, or 5, means hydrocarbon having at least n carbon atom(s) permolecule. The term “alkane” means substantially saturated compoundscontaining hydrogen and carbon only, e.g., those containing ≦1% (molarbasis) of unsaturated carbon atoms. The term alkane encompasses C₁ to C₅linear, iso, and cyclo alkanes. The term “unsaturate” means a C_(n)hydrocarbon containing at least one carbon atom directly bound toanother carbon atom by a double or triple bond. The term “PeriodicTable” means the Periodic Chart of the Elements, as it appears on theinside cover of The Merck Index, Twelfth Edition, Merck & Co., Inc.,1996.

For the purpose of this description and appended claims, electriccurrents are defined in the conventional way: the direction of electriccurrent is the direction in which positive charges move. The term“electric potential” is defined in terms of electric current flow: intraversing a resistor, for example, electric current flows from a high(more positive) electric potential to a lower (less positive, zero, ornegative) electric potential. Referring to the electric circuit 100 ofFIG. 1 a, a source of electricity 110, e.g., a primary battery, has apositive (+) terminal 120 and negative (−) terminal 130 as shown. Adevice 140 (shown as a linear device, a resistor, by way of example) isconnected between the positive and negative terminals. The electricpotential V₊ is greater than electric potential V⁻. Although V⁻ can bereferenced to an electric potential of zero volts, this is not required.For example, a bias voltage (positive or negative) can be applied to thecell, e.g., to the cell's negative terminal. An electric current isobserved to flow from terminal 120 through resistor 140, to terminal130. An electromotive force (“EMF”) is the electric potential (energy)per unit positive charge gained by a positive unit charge traversingsource 110. Consequently, the electric potential at the positiveterminal V₊ of source 110 is higher (more positive) than the potentialat the negative terminal, V⁻.

The term “applying an electric potential to the cell” means electricallyconnecting the positive terminal 120 of source 110 to the negativeterminal of an electrochemical cell, and providing an electrical returnpath for electric current flow from the cell's positive terminal to thesource's negative terminal 130. Referring to FIG. 1 b, device 140 isrepresented now by an electrochemical cell, which can be a linear ornon-linear device depending on how it is operated. The electrochemicalcell comprises an anode (negative electrode), a cathode (positiveelectrode), and a non-porous membrane located between the cell's anodeand cathode. The cell's negative terminal (the cell's anode, from whichelectrons leave the cell) has an electric potential of V_(a). The cell'spositive terminal (the cell's cathode, toward which electrons enter thecell) has an electric potential of V_(c). Electron movement is in theopposite direction of electric current flow, as shown in the figure. Afirst mixture comprising ≧90.0 wt. % methane based on the weight of thefirst mixture enters cell 140 from the left. At least a portion of themethane contacting the cell's anode is converted to CH₂ fragments andhydrogen ions. At least a portion of the hydrogen ions traverse thecell's membrane and combine at the cathode to form molecular hydrogen. Asecond mixture comprising at least a portion of (i) the molecularhydrogen, (ii) ethane and ethylene produced from CH₂ fragments, and(iii) unreacted methane is conducted away from the right hand side ofthe cell. In certain aspects, not shown, the cell is configured toprevent the flow of the first mixture from the inlet to the outlet. Forexample, sealing means, such as one or more o-rings, can be utilized forpartitioning the cell into an anode chamber and a cathode chamber,wherein (i) the anode chamber comprises the cell's anode and a firstopening 150 and (ii) the cathode chamber comprises the cell's cathodeand a second opening 160. In these aspects, (i) molecular hydrogen canbe removed from the cathode chamber via opening 160 and (ii) ethylene,ethane, and unreacted methane can be removed from the anode chamber viaopening 150. Although FIG. 1 b depicts an electrochemical cell forconverting methane to molecular hydrogen and C₂ hydrocarbons, theinvention is not limited thereto. The Figure and its description are notmeant to foreclose other aspects within the broader scope of theinvention, e.g., aspects where an oxygen-containing fluid (e.g., one ormore oxidants, such as air, molecular oxygen, etc.) are reacted withhydrogen ions at the cell's cathode to produce water.

An electrochemical cell in chemical and thermal equilibrium in an “opencircuit” configuration (electricity source 110 disconnected) generallyhas a characteristic electric potential difference between the cell'spositive and negative terminals (the “cell potential”), which depends,e.g., on the electrode materials, reactants, products, flow rates,temperature, pressure, etc. An electric potential difference V_(i)across a cell (equal to V_(a)−V_(c)) in chemical and thermal equilibriumcan be increased or decreased to establish a new electric potentialdifference V_(f) by applying an electric potential from electricitysource 110 as shown in FIG. 1 b. The invention can be practiced evenwhen the electrical behavior of cell 140 is non-ohmic, e.g., whethercell 140 operates as a linear device, a non-linear device, or somecombination thereof. The electric potential difference across a cell(V_(a)−V_(c)), can be measured using conventional means. For example,when source 110 produces direct current, a D.C. voltmeter can be used.When source 110 produces alternating current, V_(a)−V_(c) can bemeasured using an oscilloscope for example.

A substantially-constant electric potential can be applied to the cellto establish the specified electric potential differences across thecell (V₁, V₂, etc.). Alternatively, the electric potential applied tothe cell varies in time. For example, source having a periodic ornon-periodic voltage variation can be utilized, such as an alternatingcurrent source. Example of non-constant electric potential differencesacross the cell which can result from applying a time-varying electricpotential to the cell are shown in FIG. 1 c.

When the electric potential applied to the cell is not substantiallyconstant, average values for the electric potential differences acrossthe cell (V₁, V₂, etc.) can be defined as shown in FIG. 1 c. Thebaseline is referenced to the electric potential of the cell's positiveelectrode, V_(c), which can include an optional bias. V₁ is equal to thetime-average value of (V_(a)−V_(c)) over interval τ₁, and V₂ is equal tothe time-average value of (V_(a)−V_(c)) over interval τ₂. Average valuesfor V₃, V₄, etc., over corresponding intervals τ₃, τ₄, etc., can bedetermined in the same way.

FIG. 1 c illustrates how to do this when the applied electric potentialresults in establishing an electric potential difference across the cellhaving smoothly-varying and abruptly-changing waveforms. Averageelectric potential differences across the cell resulting from applyingother waveform shapes to the cell (including non-periodic waveforms) canbe obtained using conventional Fourier analysis, for example, such asthose involving a convolution of a plurality of individual sinusoidalwaveforms. Applied electric potentials can be obtained from conventionalsources of electricity, such as one or more batteries, D.C. powersupplies, A.C. power supplies, etc. Generators of radio frequency wavesand microwaves (such as one or more transmitters) can be utilized forapplying the electric potential, e.g., when at least one of τ₁ and/or τ₂is ≦1×10⁻³ seconds. Circuit efficiency is improved when the source ofelectricity has an internal impedance that is substantially the same asthat of the electrochemical cell. As an example, suitable electricitysources include those capable of applying an electric potential in therange of −100 volts to 100 volts and having an internal impedance ≦1000Ohms, such as those capable of applying an electric potential in therange of +20 volts to −20 volts, or +10 volts to −10 volts, or 0 voltsto 10 volts; and having an internal impedance ≦100 ohms, e.g., ≦10 ohms.

Control means can be utilized for maintaining the desired electricpotential difference across the cell at the desired value or values forthe desired time period. The control means can include one or more dataprocessors interfaced (i) with one or more voltage sensing means (e.g.,a voltmeter, analog-to-digital converter, etc.) and (ii) one or moremeans for applying an electric potential to the cell (e.g., one or moreA.C. or D.C. power supplies, batteries, etc.). For example, one or moredigital or analog computers can be utilized for acquiring data from avoltmeter, the voltmeter being configured to measure the electricpotential difference across the cell (e.g., V_(a)−V_(b)). The computercan be configured to automatically provide a correction signal to thepower supply when the electric potential difference across the celldiffers from the desired value. The power supply is configured torespond to the correction signal by increasing or decreasing the appliedelectric potential to the cell until the desired value is attained.Conventional data processors, voltage sensing means, power supplies, andinterfaces/interconnections can be used to do this, but the invention isnot limited thereto.

An electric potential difference across the cell can be established fora first time interval τ₁ such as by applying an electric potential tothe cell. More particularly, the duration of the first time interval,τ₁, can be selected to be a time interval approximating the time neededto form C₂H₄ species at the first electrode. Duration τ₁ may beapproximated by a number of methods known in the art, e.g., bycalculating the ratio of the moles CH_(x) per square cm of electrodesurface to the moles of CH₂ entering the cell for conversion asdetermined by the space velocity (e.g., the weight hourly spacevelocity, “WHSV”) of the CH₄ entering the cell (moles/cm·sec.), whichcan as shown in equation (1):

CH₂˜(methane feedspace velocity)·(% conversion/pass)·(electrode surfacearea)⁻¹.  (1)

The moles of CH_(x) per square cm of electrode surface can be estimatedaccording to equation (2):

CH_(x)˜(gram-atoms/cm² of catalyst)·(CH_(x) coverage/catalystsite).  (2)

By way of example, the time needed to form C₂H₄ species at an electrodehaving a surface area of 100 m²/gram, a platinum coverage of 50% of thesurface area, and a methane feed WHSV of 0.5 hr⁻¹, assuming a conversionof 25% per pass, and a CH_(x) coverage moles/catalyst site of 0.45results in an estimated value for first time interval, τ₁, of 0.1seconds.

A minimum value for the first time interval, τ_(1mim), which correspondsto the minimum time needed for two CH₂ molecules to combine to formC₂H₄, can be estimated using the formula:

$\begin{matrix}{{\text{?} = \frac{\text{?}}{\text{?}}},{\text{?}\text{indicates text missing or illegible when filed}}} & (3)\end{matrix}$

where l_(min) is the minimum mean free path and

_(max) is the root mean square velocity of CH₂ fragments in the firstelectrode chamber under specified conditions of temperature andpressure. The parameter

_(max) can be estimated from the equipartition theorem:

$\begin{matrix}{{\text{?} = \sqrt{\frac{\text{?}}{m}}},{\text{?}\text{indicates text missing or illegible when filed}}} & (4)\end{matrix}$

where m is the mass of a CH₂ fragment (2.33×10⁻²³ g), K is the Boltzmannconstant, and T is the temperature of the first electrode chamber in °Kelvin (° K). For a temperature of 200° C. (473° K),

$\begin{matrix}{\text{?} = {\text{?} \times 10^{4}{\frac{\text{?}}{\text{?}}.\text{?}}\text{indicates text missing or illegible when filed}}} & (5)\end{matrix}$

The parameter l_(min) can be estimated from the equation:

$\begin{matrix}{\mspace{79mu} {{l_{\min} = \frac{1}{\text{?}}},{\text{?}\text{indicates text missing or illegible when filed}}}} & (6)\end{matrix}$

where σ_(max) is a CH₂ fragment's maximum cross sectional area and n isthe number of CH₂ fragments per cm³. The parameter σ_(max) can beestimated using the formula:

σ_(max) =

d ²,  (7)

where d is twice the length of a C—H bond in cm. For a CH₂ fragment,σ_(max) is approximately 1.41×10⁻¹⁵ cm².

The parameter n can be estimated from the equation of state:

$\begin{matrix}{\mspace{79mu} {{n = \frac{\text{?}}{\text{?}}},{\text{?}\text{indicates text missing or illegible when filed}}}} & (8)\end{matrix}$

where P is the first electrode chamber pressure.

At a temperature of 200° C. (473° K) and a pressure of 1.0 atmosphere(10⁶ dynes/cm²), n is approximately 1.5×10¹⁷ CH₂ molecules per cm³,l_(min) is approximately 3.35×10⁻⁵ cm, and τ_(1mim) is approximately3.63×10⁻¹⁰ sec., i.e., the estimated value of the minimum time neededfor two CH₂ fragments to interact and form C₂H₄ is on the order of1.0×10⁻¹⁰ sec.

The maximum value of the first time interval, τ_(1max), is preferably onthe order of the time, τ_(c), which is the time needed to successivelyoxidize methane in the first electrode chamber to coke. Thus, preferablyτ_(1max)≦10.0·τ_(c), e.g., τ_(1max)≦1.0·τ_(c), such as τ₁≦0.1·τ_(c). Inparticular aspects, τ₁≦τ_(CH), wherein τ_(CH) is a time needed toproduce on the first electrode 1.0×10⁻⁹ mole cm⁻² of carbon atoms fromCH fragments during each application of the electric potentialdifference across the cell for a first time interval τ₁.

Thus, in some aspects, first time interval, τ₁, may be from 1.0×10⁻¹⁰ to10.0 sec. The lower limit on the first time interval, τ₁, may beselected from values of e.g., 1.0×10⁻¹⁰ sec., 1.0×10⁻⁹ sec., 1.0×10⁻⁸sec., 1.0×10⁻⁷ sec., 1.0×10⁻⁶ sec., 1.0×10⁻⁵ sec., 1.0×10⁻⁴ sec.,1.0×10⁻³ or 1.0×10⁻² sec. The upper limit on the first time interval,τ₁, may be selected from values of e.g., 10.0 sec., 5.0 sec., 2.5 sec.,1.0 sec., 0.5 sec., 0.25 sec., 0.10 sec., 0.05 sec., 0.02 sec., 0.01sec., 1.0×10⁻³ sec., 1.0×10⁻⁴ sec., 1.0×10⁻⁵ sec., 1.0×10⁻⁶ sec.,1.0×10⁻² sec., 1.0×10⁻⁸ sec., 1.0×10⁻⁹ sec. In particular embodiments,the first time interval, τ₁, may be 1.0×10⁻¹⁰ to 1.0 sec., 1.0×10⁻¹⁰ to0.1 sec., or 1.0×10⁻¹⁰ to 0.01 sec., 1.0×10⁻¹⁰ to 1.0×10⁻⁸ sec., or1.0×10⁻⁹ to 1.0×10⁻⁸ sec. Other exemplary ranges for the first timeinterval, τ₁, include 0.05 sec. to 0.25 sec., 0.01 to 0.2 sec., or 0.001to 0.1 sec. In other embodiments the first time interval, τ₁, may be1.0×10⁻¹⁰ to 1.0×10⁻⁶ sec.

Optionally, the duration of the first interval is regulated so that itis less than the time needed to form an appreciable amount of carbon atthe first electrode. Optionally, the duration of the second timeinterval is regulated so that an appreciable amount of ethylene can formfrom the interaction of the CH₂ fragments, and so that carbon that hasformed at the first electrode can react with H⁺ and e⁻ to restore CH_(x)fragments on the first electrode surface. Preferably the duration of thefirst and second intervals are selected and regulated so that two ormore of these desired effects can occur.

The temperature and pressure of the first mixture when an electricpotential difference is established across the cell during a first timeinterval is not critical. In particular embodiments, however, improvedresults may be achieved where the temperature of the first mixture isfrom 100° C. to 500° C. (e.g., 1.0×10²° C. to 5.0×10²° C.). In someembodiments, lower limit on the range of temperature of the firstmixture during that application of the electric potential difference forthe first time interval may be 100° C., 125° C., 150° C., 175° C., 200°C., 225° C., 250° C., 300° C., 350° C., 375° C., 400° C., 450° C., 475°C., 480° C., 490° C., or 495° C. The upper limits on the range oftemperatures of the first mixture may be 125° C., 150° C., 175° C., 200°C., 225° C., 250° C., 300° C., 350° C., 375° C., 400° C., 450° C., 475°C., 480° C., 490° C., 495° C., or 500° C. Any upper limit may be matchedwith any lower limit; e.g., 100° C. to 300° C., 200° C. to 450° C. or180° C. to 225° C. A stream at such temperature may be at a pressure of0.1 bar to 100.0 bar, preferably 1.0 to 50.0 bar, 5.0 to 50.0 bar, 1.0to 25.0 bar, 5.0 to 25.0 bar, 1.0 to 20.0 bar, 5.0 to 20.0 bar, 1.0 to15.0 bar, 5.0 to 15.0 bar, 1.0 to 10.0 bar, 5.0 to 10.0 bar 1.0 to 5.0bar, or 1.0 to 2.5 bar. Such temperatures and pressures typically referto the temperature and pressure of the first electrode chamber.

In particular aspects, the electric potential difference establishedacross the cell during the first time interval, τ₁, is selected so thatthe current density, I₁ through the membrane during the first timeinterval, τ₁, achieves a value of 1.0 mA cm⁻² to 100.0 mA cm⁻². Thelower limit on the current density I₁ may be 1.0 mA cm⁻², 2.0 mA cm⁻²,5.0 mA cm⁻², 7.5 mA cm⁻², 10.0 mA cm⁻², 15.0 mA cm⁻², 20.0 mA cm⁻², 25.0mA cm⁻², 30.0 mA cm⁻², 40.0 mA cm⁻², 50.0 mA cm⁻², 60.0 mA cm⁻², 70.0 mAcm⁻², 80.0 mA cm⁻², 85.0 mA cm⁻², 90.0 mA cm⁻², 95.0 mA cm⁻², or 99.0 mAcm⁻². An upper limit on the range of the current density I₁ may be 2.0mA cm⁻², 5.0 mA cm⁻², 7.5 mA cm⁻², 10.0 mA cm⁻², 15.0 mA cm⁻², 20.0 mAcm⁻², 25.0 mA cm⁻², 30.0 mA cm⁻², 40.0 mA cm⁻², 50.0 mA cm⁻², 60.0 mAcm⁻², 70.0 mA cm⁻², 80.0 mA cm⁻², 85.0 mA cm⁻², 90.0 mA cm⁻², 95.0 mAcm⁻², 99.0 mA cm⁻², or 100.0 mA cm⁻². Any upper limit may be matchedwith any lower limit; e.g., preferably 5.0 mA cm⁻² to 75.0 mA cm⁻² or15.0 mA cm⁻² to 50.0 mA cm⁻². Such current densities may be achieved byestablishing an electric potential difference across the cell that is inthe range of 0.1 to 20.0 V, particularly 1.0 to 20.0 V, 0.1 to 15.0 V,1.0 to 15.0 V, 0.1 to 10.0 V, 5.0 to 10.0 V, 1.0 to 10.0 V, 0.1 to 5.0V, or 1.0 to 5.0 V across the cell during the first time interval, τ₁.

After the cell is operated at the operating electric potentialdifference for the first time interval, τ₁, the electric potentialdifference across the cell is lessened, reversed, or substantiallyremoved for a second time interval, τ₂. The second time interval, τ₂, istypically is selected such that τ₂≦τ₁. For example, τ₂ may be 0.01·τ₁ to0.9·τ₁, 0.05·τ₁ to 0.8·τ₁, 0.1·τ₁ to 0.7·τ₁, 0.2·τ₁ to 0.6·τ₁, 0.3·τ₁ to0.5·τ₁, particularly τ₂=0.01·τ₁ to 0.5·τ₁, 0.01·τ₁ to 0.25·τ₁, 0.01·τ₁to 0.1·τ₁.

The electric potential difference during the second time interval istypically maintained in a range such that substantially no oxidation ofCH to carbon occurs. Thus, the value of the electric potentialdifference across the cell during the second time interval, V₂, isgenerally less than the electric potential difference across the cellduring the first time interval, V₁. Optionally, V₁ and V₂ are related asfollows:

-   -   (i) V₁>0, V₂≧0, and V₂=A₁·V₁, where A₁ is in the range of from        about 0.1 to about 0.9, such as in the range of from about 0.2        to about 0.8;    -   (ii) V₁≧0 and V₂<0; or    -   (iii) V₁<0, V₂<0, and |V₂|=A₂|V₁|, where A₂≧1.1, e.g., ≧2, such        as ≧10. Optionally, V₁≧0, and |V₂|<|V₁|. In certain embodiments        where V₁ is positive, V₂=−1.0·V₁ to 0.1·V₁, or 0 to 0.1·V₁.        Alternatively, the electric potential difference may be selected        to produce a second current density through the cell, I₂, where        I₂ is in the range of −10.0·I₁ to 0.9·I₁.

Optionally, V₂ and V₃ can be related as follows:

-   -   (i) V₃>0, V₂≧0, and V₂=A₃·V₃, where A₃ is in the range of from        about 0.1 to about 0.9, such as in the range of from about 0.2        to about 0.8;    -   (ii) V₃≧0 and V₂<0; or    -   (iii) V₃<0, V_(2<0), and |V₂|=A₄|V₃| where A₄≧1.1, e.g., ≧2,        such as ≧10. Optionally, V₃≧0, and |V₂|<|V₃|. In certain        embodiments, V₃ is substantially the same as V₁, τ₃ is        substantially the same as τ₁, and I₃ is substantially the same        as I₁.

Optionally, V₃ and V₄ are related as follows: (i) V₃>0, V₄≧0, andV₄=A₅·V₃, where A₅ is in the range of from about 0.1 to about 0.9, suchas in the range of from about 0.2 to about 0.8;

-   -   (ii) V₃≧0 and V₄<0; or    -   (iii) V₃<0, V₄<0, and |V₄|=A₆|V₃|, where A₆≧1.1, e.g., ≧2, such        as ≧10. Optionally, V₃≧0, and |V₄|<|V₃|. In certain aspects, V₄        is substantially the same as V₂, τ₄ is substantially the same as        τ₂, and I₄ is substantially the same as I₂.

Methane can be continuously converted to C₂₊ unsaturates at the firstelectrode without producing an appreciable amount of carbon on the firstelectrode. Typically, the process is operated cyclically, by applying analternating electric potential to the cell to establish an alternatingelectric potential difference across the cell. The electric potentialdifference across the cell can be in the range of from about 0.1 rmsvolts to 20.0 rms volts at a frequency (in Hertz)≧(2·π·τ_(min))⁻¹. Theshape of the electric potential difference wave-form can be, e.g., oneor more of sinusoidal, trapezoidal, rectangular, and combinationsthereof. Symmetric waveforms (V₁ substantially the same as V₂, τ₁substantially the same as τ₂) can be utilized. Optionally, the waveformis modulated by, e.g., a linear or non-linear function, in amplitude,frequency, or a combination thereof. Optionally, the waveform issymmetric about an axis that is substantially equal to zero volts.

Optionally, at least a portion of the C₂₊ unsaturates may be conductedaway from the first electrode, particularly at a rate ≧1.0×10⁻⁷ molemin·⁻¹ cm⁻², preferably 1.0×10⁻⁷ mole min·⁻¹ cm⁻² to 1.0×10⁻⁵ molemin·⁻¹ cm⁻². In particular aspects, the rate is 5.0×10⁻⁶ mole min·⁻¹cm⁻² to 5.0×10⁻⁵ mole min·⁻¹ cm⁻² or 1.0×10⁻⁶ mole min·⁻¹ cm⁻² to0.5×10⁻⁶ mole min·⁻¹ cm⁻². The C₂₊ unsaturates may be conducted awayeither continuously or periodically. In particular embodiments, the C₂₊unsaturates are conducted away periodically for ≧100.0 minutes,preferably 100.0 to 1000.0 minutes or more. In some aspects, the lowerlimit on the period that the C₂₊ unsaturates may be conducted away fromthe first electrode may be 150.0 mins., 200.0 mins., 250.0 mins., 300.0mins., 350.0 mins., 400.0 mins., 450.0 mins., 500.0 mins., 550.0 mins.,600.0 mins., 650.0 mins., 700.0 mins., 750.0 mins., 800.0 mins., 850.0mins., 900.0 mins., or 950.0 mins. Such lower limit may be coupled withan upper limit on the period of 200.0 mins., 250.0 mins., 300.0 mins.,350.0 mins., 400.0 mins., 450.0 mins., 500.0 mins., 550.0 mins., 600.0mins., 650.0 mins., 700.0 mins., 750.0 mins., 800.0 mins., 850.0 mins.,900.0 mins., 950.0 mins., 1000.0 mins., 2000.0 mins., or 2500.0 mins.Any upper limit may be matched with any lower limit; e.g., 150.0 to950.0 mins., 200.0 to 900.0 mins., 250.0 to 850.0 mins., 300.0 to 800.0mins., 350.0 to 750.0 mins., 800.0 to 1000.0 mins.

Optionally, the processes described herein may also further includeconducting water away from the first electrode and/or the secondelectrode. The water may be conducted away either continuously orperiodically. In particular aspects, the water is conducted awayperiodically for ≧100.0 minutes, preferably 100.0 to 1000.0 minutes ormore. In some embodiments, the lower limit on the period water isconducted away from the first electrode may be 150.0 mins., 200.0 mins.,250.0 mins., 300.0 mins., 350.0 mins., 400.0 mins., 450.0 mins., 500.0mins., 550.0 mins., 600.0 mins., 650.0 mins., 700.0 mins., 750.0 mins.,800.0 mins., 850.0 mins., 900.0 mins., or 950.0 mins. Such lower limitmay be coupled with an upper limit on the period of 200.0 mins., 250.0mins., 300.0 mins., 350.0 mins., 400.0 mins., 450.0 mins., 500.0 mins.,550.0 mins., 600.0 mins., 650.0 mins., 700.0 mins., 750.0 mins., 800.0mins., 850.0 mins., 900.0 mins., 950.0 mins., 1000.0 mins., 2000.0mins., or 2500.0 mins. Any upper limit may be matched with any lowerlimit; e.g., 150.0 to 950.0 mins., 200.0 to 900.0 mins., 250.0 to 850.0mins., 300.0 to 800.0 mins., 350.0 to 750.0 mins., 800.0 to 1000.0 mins.

Optionally, aspects of the processes described herein may furtherinclude conducting an oxygen-containing mixture to the electrochemicalcell's cathode when an electric potential difference V₁ has beenestablished across the cell during the first time interval, τ₁. Theoxygen-containing mixture preferably comprises ≧10.0 wt. % molecularoxygen (O₂), based on the weight of the oxygen-containing mixture.Optionally, the oxygen-containing mixture may be air or comprise 10.0 to100.0 wt. % O₂, particularly 10.0 to 99.0 wt. % O₂, 20.0 to 90.0 wt. %O₂. The lower limit on range of O₂ in the oxygen-containing mixture maybe 12.5 wt. %, 15.0 wt. %; 20.0 wt. %, 25.0 wt. %, 30.0 wt. %, 40.0 wt.%, 50.0 wt. %, 60.0 wt. %, 70.0 wt. %, 75.0 wt. %, 80.0 wt. %, 90.0 wt.%, 95.0 wt. %, or 99.0 wt. %. Such lower limit may be coupled with anupper limit on the O₂ content of 15.0 wt. %; 20.0 wt. %, 25.0 wt. %,30.0 wt. %, 40.0 wt. %, 50.0 wt. %, 60.0 wt. %, 70.0 wt. %, 75.0 wt. %,80.0 wt. %, 90.0 wt. %, 95.0 wt. %, 99.0 wt. %, or 100.0 wt. %. Anyupper limit may be matched with any lower limit.

C₂₊ unsaturates formed in aspects of the processes described herein maybe further reacted by any number of suitable reaction types, e.g.,oligomerization, polymerization, hydroxylation, and/or hydrogenation.

The configuration of the electrocatalytic apparatus for the processesdescribed above is not critical provided that it is capable ofconverting at least a portion of a methane-containing stream to C₂ ⁺unsaturates. FIG. 5 illustrates a schematic representation of anapparatus 500 for the electrocatalytic conversion of hydrocarbonsaccording to an embodiment of the invention. Apparatus 500 includes anelectrochemical cell 510. The electrochemical cell 510 is connected viaelectrical contacts 520 to a source of electricity 530 capable ofapplying an electric potential to the cell sufficient for establishingan electric potential difference across the cell. The electric potentialdifference established across the cell causes an electric current toflow through the cell, which current and electric potential differenceactivate the formation of CH₃ fragments from methane. The electricpotential applied to the cell (and the current flowing through the cell)can be regulated by providing electricity source 530 with voltageregulation means 540 (e.g., a voltage regulator, such as a conventionalthree-terminal regulator). FIG. 5 shows voltage regulator 540 connectedin series with electricity source 530, but this is not required, and incertain embodiments one or more voltage regulators are connected withelectricity source 530 in series, parallel, or a with a combination ofseries and parallel connections. In particular embodiments, voltageregulator 530 is integral with the power supply 530. The voltageregulator 540 is configured to apply a desired electric potential tocell 510 to produce the desired electric potential difference across thecell (and/or a desired electric current through the cell 510) for afirst time interval of duration τ₁. The voltage regulator 540 isconfigured to, after the time τ₁, to lessen, reverse, or substantiallyremove the supply of power to the cell 510 for a second time interval ofduration τ₂. Current regulation (not shown) can be used with or as analternative to voltage regulation.

Optionally, apparatus 500 may also include conduit 550 fluidly connectedto a separation unit 560 for separating C₂₊ compounds formed in the cell510 from reactant gases. Apparatus 500 may also include recycle conduit570 to return unreacted reactant gases to the cell 510 and conduit 580configured to send C₂₊ compounds, particularly C₂ ⁺ unsaturates forfurther processing, e.g., hydrogenation, oligomerization, polymerizationhydroxylation, etc.

FIG. 6 illustrates an aspect of the electrochemical cell 510 of FIG. 5.Electrochemical cell 600 includes first electrode chamber 610, e.g.,anode chamber, having a first electrode, e.g., an anode, 620. The firstelectrode chamber 610 is configured with means to receive the firstmixture comprising ≧1.0 wt. % methane, ethane, or methanol in contactwith the electrode (based on the weight of the first mixture) via inlet630. First electrode chamber 610 may also include one or more outlets640 including means for withdrawing higher hydrocarbons from the firstelectrode chamber 610. The electrochemical cell 600 also includes asecond electrode chamber, e.g., a cathode chamber, 650 having a secondelectrode, e.g., a cathode, 660 therein. The second electrode chamber650 is optionally configured to receive an oxygen-containing mixturecomprising ≧10.0 wt. % molecular oxygen (based on the weight of theoxygen-containing mixture) through inlet 670. Second electrode chamber650 typically also includes means for withdrawing hydrogen and/or watertherefrom via outlet 680. A membrane 690 is provided between the firstelectrode chamber 610 and the second electrode chamber 650. The membrane690 is configured to provide diffusive and electrical contact betweenthe first and second electrodes. Apparatus 500 may also optionallyinclude means for removing heat from the second electrode chamber 650.Such means may include, e.g., a heat exchanger or a sweep gas supply. Ifdesired, sealing means (not shown) such as o-rings can be utilized forsubstantially preventing the flow (or diffusion) of reactants and/orproducts between the electrodes and the inner surfaces of the electrodechambers.

FIG. 7 schematically illustrates an alternative apparatus 700 for theelectrocatalytic conversion of hydrocarbons according to an embodimentof the invention. In apparatus 700, power supply 710 and resistor 720form a potentiometer 730. Potentiometer 730 is electrically connected toelectrochemical cell 740, which may be of the type illustrated in FIG.6. In such a configuration, potentiometer 730 can be adjusted to applyan electric potential to the cell in order to establish an electricpotential difference across cell 740. Thus, potentiometer 730 may beadjusted to establish a control (e.g., automatically control) onelectric potential difference across the cell (V₁) and an amount ofelectric current flowing through the cell (I₁) for the desired firsttime interval duration τ₁ and thereafter lessen, reverse, orsubstantially remove the electric potential applied to cell 740 in orderto establish a second electric potential difference across the cell (V₂)and a second amount of electric current flowing through the cell (I₂)for a desired second time interval τ₂. Apparatus 700 may also includeconduit 750 fluidly connecting the anode chamber 755 to separation unit760 for separating C₂₊ compounds formed in the cell 740. Separation unit760 is configured to return unreacted reactant gases to the cell 740 viarecycle conduit 770. Conduit 780 configured to send C₂₊ compounds,particularly C₂₊ unsaturates for further processing, e.g.,hydrogenation, oligomerization, polymerization hydroxylation, etc.Apparatus 700 may also include an oxygen-containing-gas supply 790 influid communication with the cathode chamber 795 of cell 740. Aspects ofthe invention illustrated in FIG. 7 are not limited to those utilizingpassive components (e.g., battery variable resistor) as shown in thefigure. Active components and combinations of active and passivecomponents for providing the indicated potentiometric functionality canbe used instead.

The first electrode typically comprises at least one metal selected fromGroup VIII of the Periodic Table. For example, the second electrode maycomprise ≧1.0 wt. % of at least one noble metal based on the weight ofthe first electrode. The first electrode may include compressed carbonpowder loaded with noble metal, carbon cloths supporting noble metal,nickel mesh impregnated with noble metal, etc. Among particularly usefulnoble metals, include Ag, Pt, Pd, Ru, etc. The first electrode may be aparticular surface of a noble metal electrode, e.g., Ni(111), Pd(111),and Pt(111). In particular embodiments, the first electrode is theanode.

The second electrode typically comprises at least one metal selectedfrom Group VIII of the Periodic Table. For example, the second electrodemay comprise ≧1.0 wt. % of at least one noble metal based on the weightof the second electrode. Like the first electrode, the second electrodemay include compressed carbon powder loaded with noble metal, carboncloths supporting noble metal, nickel mesh impregnated with noble metal,etc. Among particularly useful noble metals, include Ag, Pt, Pd, Ru,etc. In particular embodiments, the second electrode may be a particularsurface of a noble metal electrode, e.g., Ni(111), Pd(111), and Pt(111).The composition and structure of the second electrode may be the same ordifferent composition or surface than that of the first electrode. Inparticular embodiments, the second electrode is the cathode.

The electrochemical cell generally comprises at least one membrane,e.g., at least one non-porous membrane, located between the first andsecond electrode. When more than one membrane is used, the membranes canbe configured as a membrane assembly located between the first andsecond electrodes, each membrane of the assembly being in face-to-facecontact with its neighboring membrane. Although all the membranes of themembrane assembly can be of the same composition and have the samethermal and transport properties, this is not required, and in certainembodiments the membranes comprising the membrane assembly havedifferent compositions and properties. In particular embodiments, themembrane utilized in the cell of apparatus 400 comprises solid,proton-conducting membrane separating the first electrode chamber fromthe second chamber, particularly a non-porous membrane. As used herein,the term “non-porous” means that the membrane is impervious to gaseousdiffusion of methane into from one chamber to the other under zero cellcurrent and up to 2 times the operational feed-gas pressure. Themembrane provides diffusive and electrical contact between the first andsecond electrodes. Any suitable membrane may be used. Particularmembranes capable of transferring the H⁺ from the first electrodechamber, e.g., the anode chamber, to the second electrode chamber, e.g.,the cathode chamber, include, but are not limited to, phosphoric acid,perfluorosulfonic acid and/or polybenzimidazole. Further details of theuse of perfluorosulfonic acid and/or polybenzimidazole membranes aredescribed in U.S. Pat. No. 6,294,068. Other membranes are ceramics,e.g., SrCe_(0.95)Yb_(0.05)O₃ as described in Electrochemical MethaneCoupling Using Protonic Conductors; S. Hamakawa, et. al.; J.Electrochem. Soc., 40(2), 459-463 (1993).

Although the invention can utilize one electrochemical cell, it is notlimited thereto, and the preceding description is not meant to forecloseother embodiments within the broader scope of the invention, such asthose utilizing a plurality of electrochemical cells connected inseries, parallel, or series-parallel.

A representative electrochemical cell can comprise:

-   -   (i) a first electrode chamber, the first electrode chamber,        comprising a first electrode, the first electrode chamber        configured to receive a first mixture comprising ≧1.0 wt. %        hydrocarbon based on the weight of the first mixture;    -   (ii) a second electrode chamber comprising a second electrode,        said second electrode chamber configured to receive an        oxygen-containing mixture comprising ≧10.0 wt. % molecular        oxygen, based on the weight of the oxygen-containing mixture;        and    -   (iii) at least one non-porous membrane configured to provide        diffusive and electrical contact between the first electrode and        the second electrode.        The electrochemical cell can further comprise a source of        electricity in electrical communication with the electrochemical        cell for applying an electric potential to the cell; and a        voltage regulator configured to adjust the applied electric        potential to establish a first electric potential difference, V₁        across the electrochemical cell for a first time interval of        duration τ₁ and a second electric potential difference, V₂        across the electrochemical cell for a second time interval of        duration τ₂. Optionally, τ₂≦τ₁ and V₂ is more negative or less        positive than V₁.

All patents, test procedures, and other documents cited herein,including priority documents, are fully incorporated by reference to theextent such disclosure is not inconsistent with this invention and forall jurisdictions in which such incorporation is permitted. Should thedisclosure of any of the patents and/or publications that areincorporated herein by reference conflict with the present specificationto the extent that it might render a term unclear, the presentspecification shall take precedence.

As is apparent from the foregoing general description and the specificembodiments, while forms of the invention have been illustrated anddescribed, various modifications can be made without departing from thespirit and scope of the invention. Accordingly, it is not intended thatthe invention be limited thereby. Many variations will suggestthemselves to those skilled in this art in light of the above detaileddescription. All such variations are within the full intended scope ofthe appended claims. Certain embodiments and features have beendescribed using a set of numerical upper limits and a set of numericallower limits. It should be appreciated that ranges from any lower limitto any upper limit are contemplated unless otherwise indicated. Certainlower limits, upper limits and ranges appear in one or more claimsbelow. All numerical values are “about” or “approximately” the indicatedvalue, and take into account experimental error and variations thatwould be expected by a person having ordinary skill in the art.

What is claimed is:
 1. An electrochemical conversion method, comprising,(a) providing an electrochemical cell, the electrochemical cellcomprising a first electrode, a second electrode, and at least onemembrane located therebetween; (b) providing a first mixture comprising≧1.0 wt. % hydrocarbon based on the weight of the first mixture to thefirst electrode of the electrochemical cell; (c) applying an electricpotential to the cell to establish an electric potential difference V₁,across the cell during a first time interval, the first time intervalhaving a duration τ₁; (d) changing the applied electric potential to (i)establish a second electric potential difference, V₂, across the cellfor a second time interval τ₂, and (ii) produce C₂₊ unsaturatesproximate to the first electrode, wherein V₂ is more negative or lesspositive than V₁ and τ₂≦τ₁; and (e) repeating steps (c) and (d).
 2. Themethod of claim 1, wherein the hydrocarbon is methane and wherein theprocess further comprises conducting at least a portion of the C₂₊unsaturates away from the first electrode at a rate ≧1.0×10⁻⁷ molemin·⁻¹ cm⁻² for ≧100.0 minutes.
 3. The method of claim 1, wherein duringat least step (c) the first mixture is exposed to a temperature in therange of from 1.0×10²° C. to 5.0×10²° C. at a pressure in the range offrom 0.1 bar to 100.0 bar.
 4. The method of claim 1, further comprisingconducting an oxygen-containing mixture to the second electrode duringat least step (c), the oxygen-containing mixture comprising ≧10.0 wt. %molecular oxygen, based on the weight of the oxygen-containing mixture,and conducting water away from the first electrode and/or the secondelectrode.
 5. The method of claim 1, wherein the first electrode and/orthe second electrode comprises ≧1.0 wt. % of at least one noble metal.6. The method of claim 1, wherein the current density through themembrane during step (c) achieves a value in the range of from 1.0 mAcm⁻² to 100.0 mA cm⁻².
 7. The method of claim 1, wherein V₁ is in therange of from 0.1 to 20 Volts.
 8. The method of claim 1, wherein V₁ andV₂ satisfy at least one of: (i) V₁>0, V₂≧0, and V₂=A₁·V₁, wherein A₁ isin the range of from about 0.00 to about 0.99; (ii) V₁≧0 and V₂<0; or(iii) V₁<0, V₂<0, and |V₂|=A₂|V₁|, wherein A₂≧1.01.
 9. The method ofclaim 1, wherein τ₁ is in the range of from 1.0×10⁻¹⁰ to 10.0 sec. 10.The method of claim 1, wherein τ₂ is in the range of from 0.01·τ₁ to0.9·τ₁.
 11. The method of claim 1, further comprising a step ofoligomerization, polymerization, hydroxylation, or hydrogenation of atleast a portion of the C₂₊ unsaturates.
 12. The method of claim 1,wherein, the first electrode is an anode and the second electrode is acathode.
 13. The method of claim 1, wherein applying an electricpotential to the cell includes applying an external positive voltage tothe first electrode.
 14. A method of reducing coke formation in theelectrochemical production of C₂₊ unsaturates in an electrochemical cellcomprising a first electrode, a second electrode, and at least onemembrane situated between the first and second electrodes, the methodcomprising: (a) providing to the first electrode of the electrochemicalcell a first mixture comprising ≧1.0 wt. % methane based on the weightof the first mixture; (b) applying an electric potential difference tothe electrochemical cell to establish a first electric potential V₁across the electrochemical cell sufficient to initiate the conversion ofat least a portion of the methane during a first time interval ofduration τ₁; (c) establishing a second electric potential difference V₂across the cell during a second time interval of duration τ₂, whereinτ₂≦τ₁; and (d) repeating steps (b) and (c), and at least during step (c)producing C₂₊ unsaturates.
 15. The method of claim 14, furthercomprising conducting at least a portion of the C₂₊ unsaturates awayfrom the first electrode at a rate ≧1.0×10⁻⁷ mole min·⁻¹ cm⁻² for ≧100.0minutes.
 16. The method of claim 14, wherein during step (c) the firstmixture is exposed to a temperature in the range of from 100° C. to 500°C. at a pressure in the range of from 0.1 bar to 100.0 bar.
 17. Themethod of claim 14, further comprising conducting an oxygen-containingmixture to the second electrode during step (c), the oxygen-containingmixture comprising ≧10.0 wt. % molecular oxygen and conducting wateraway from the first electrode and/or the second electrode.
 18. Themethod of claim 14, wherein the first electrode and/or the secondelectrode comprises ≧1.0 wt. % of at least one noble metal.
 19. Themethod of claim 14, wherein the current density through the membraneduring step (c) achieves a value in the range of from 1.0 mA cm⁻² to100.0 mA cm⁻².
 20. The method of claim 14, wherein V₁ achieves a valuein the range of from 0.1 Volts to 20 Volts during step (b).
 21. Themethod of claim 14, wherein V₁ and V₂ satisfy at least one of: (i) V₁>0,V₂≧0, and V₂=A₁·V₁, wherein A₁ is in the range of from about 0.00 toabout 0.99; (ii) V₁≧0 and V₂<0; (iii) V₁<0, V₂<0, and |V₂|=A₂|V₁|,wherein A₂≧1.01; (iv) V₂ is more negative or less positive than V₁; or(v) V₁≧0, and |V₂|<|V₁|.
 22. The method of claim 14, wherein τ₁ is inthe range of from 1.0×10⁻¹⁰ to 10.0 sec.
 23. The method of claim 14,wherein τ₂ is in the range of from 0.01·τ₁ to 0.9·τ₁.
 24. The method ofclaim 14, further comprising a step of oligomerization, polymerization,hydroxylation, or hydrogenation of at least a portion of the C₂₊unsaturates.
 25. A method for the electrochemical conversion of carbonto C₂₊ unsaturates comprising: (a) providing a first mixture to a firstelectrode of an electrochemical cell, the first mixture comprising ≧1.0wt. % methane based on the weight of the first mixture, wherein (i) theelectrochemical cell comprises the first electrode, a second electrode,and at least one membrane located therebetween, and (ii) the firstelectrode having carbon deposits formed from the methane feed; (b)establishing a first electric potential difference V₁ across theelectrochemical cell, the first electric potential difference beingsufficient to initiate the conversion of at least a portion of the firstelectrode's carbon during a first time interval of duration τ₁; (c)establishing a second electric potential difference V₂ across the cellduring a second time interval of duration τ₂, wherein τ₂≦τ₁; and (d)repeating steps (b) and (c) and producing C₂₊ unsaturates during atleast step (c).